Bidirectional power converter for balancing state of charge among series connected electrical energy storage units

ABSTRACT

A system is provided for balancing state of charge among plural series connected electrical energy storage units that includes a power converter that selectively couples to an individual storage unit of the a string of electrical energy storage units and transfers energy by bidirectionally between the individual storage unit and the string of storage units.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/464,391, filed Apr. 18, 2003 and U.S. Provisional Application No.60/428,666, filed Nov. 25, 2002. The entire teachings of the aboveapplications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Hybrid electric vehicles (HEVs) combine the internal combustion engineof a conventional vehicle with the battery and electric motor of anelectric vehicle. This combination offers the driving range and rapidrefueling features to which consumers are accustomed with conventionalvehicles, while achieving improved fuel economy and lower emissions.

Typical HEV designs such as those already on the market and those whichwill be introduced shortly, are so-called “parallel” configurations. Inthe parallel HEV, the battery powered motor is principally used to boostengine torque for hill climbing and high acceleration demands. Whenboost torque is not required, the engine drives the electric motor as agenerator to recharge the battery. The motor is also driven as agenerator during braking events, thus, relieving thermal loading of theconventional friction brakes and enabling recovery of vehicle kineticand potential energy which is returned to the battery.

The battery “pack” of a typical HEV consists of one or more “modules” ofseries-connected “cells.” Nickel cadmium (NiCad) and nickel metalhydride (NiMH) cells have been successfully employed in recentlyintroduced HEVs while higher performance lithium ion (Li-ion) cells areenvisioned for future generation designs.

Desirable attributes of battery cells for HEV applications are high-peakspecific power, high specific energy at pulse power, fast chargeacceptance to maximize regenerative braking utilization, and longcalendar and cycle life. Achieving a favorable HEV battery pack lifetimerequires some means to monitor cell state of charge (SOC) and control ofcell charging and discharging to assure that all cells in the pack arewell “balanced” or “equalized,” for example, at a nominally uniformstate of charge. The development of means to achieve affordable andreliable balanced cell operation, especially for newer Li-ion cells, haspresented significant technical challenges Lithium ion batteries are nowwidely used in laptop computer and cell-phone products because of theirhigh specific energy. They also have high specific power, high energyefficiency, good high-temperature performance, and low self-discharge.

Components of lithium ion batteries could also be recycled. Thesecharacteristics make lithium ion batteries desirable for HEVapplications. However, to make them commercially viable for HEVs,further development is needed to improve calendar and cycle life andcost.

SUMMARY OF THE INVENTION

If lithium-ion batteries are to be successfully employed in HEVapplications, the state of charge of individual battery cells will needto be continuously balanced to maintain a high cell calendar life andcell capacity. Cells must have their state of charge equalized toward atarget state of charge so they uniformly support HEV operation.Furthermore, care must be taken to assure that an individual cell is notcharged beyond its safe limit. State of charge may be determined from anopen circuit cell voltage measurement, or under load, from a measurementof cell voltage combined with cell impedance and current.

Despite the performance advantages of lithium-ion battery technology,there is a cost tradeoff associated with increased complexity of thecontrols required to equalize the battery state of charge. Theachievement of an affordable solution is particularly challenging in thecase of very long high voltage series strings of cells required for anHEV.

According to one aspect of the invention, a bidirectional powerconverter is provided for balancing state of charge among seriesconnected electrical energy storage units. Cell chemistries other thanlithium-ion, such as nickel-cadmium, lead-acid and nickel metal hydride,may also benefit from embodiments of the bi-directional power converter.

According to one embodiment, a system for balancing state of chargeamong plural series connected electrical energy storage units includes apower converter that selectively couples to an individual storage unitof the a string of electrical energy storage units and transfers energybidirectionally between the individual storage unit and the string ofstorage units.

In particular embodiments, the power converter can transfer energy at acontrollable rate of transfer. The power converter can monitor voltageand current data of the individual storage unit resulting from thetransfer of energy. The power converter can transfer units of energybetween the individual storage unit and the string of storage units.

In particular embodiments for transferring energy from the individualstorage unit to the string of storage units, the power converterincludes (i) a primary inductor; (ii) a first secondary inductormagnetically coupled to the primary inductor; and (iii) a first switchselectively coupling the individual storage unit to the primaryinductor. The first secondary inductor further couples to an outputcapacitor that is coupled in parallel to the string of storage units.When the first switch is on, energy is transferred from the individualstorage unit to charge the primary inductor. When the first switch isoff, the energy is discharged into the first secondary inductor tocharge the output capacitor, which discharges the energy to the stringof storage units.

In particular embodiments, the system can include a first pulsegenerator that provides first enable signals to the first switch. Thefirst switch couples the individual storage unit to the primary inductorin response to the first enable signals, resulting in energy beingtransferred from the individual storage unit to the string of storageunits.

The system can also include a second pulse generator that providessecond enable signals to the first pulse generator. The second enablesignals control the transfer of energy from the individual storage unitto the string of storage units at a controllable rate with the firstpulse generator providing first enable signals in response to the secondenable signals.

In particular embodiments, the system can further include a secondsecondary inductor that is coupled to the individual storage unit withthe second secondary inductor having a secondary voltage. A voltagecomparator receives the secondary voltage and a reference voltage at itsinputs. When the secondary voltage is greater than the referencevoltage, the second pulse generator is activated. When the secondaryvoltage reaches the references voltage, the second pulse generator isdeactivated.

In particular embodiments for transferring energy from the string ofunits to the individual storage unit, the system can include (i) aprimary inductor; (ii) a first secondary inductor magnetically coupledto the primary inductor; and (iii) a second switch selectively couplingthe first secondary inductor to the string of storage units. When thesecond switch is on, energy is transferred from the string of storageunits to charge the first secondary inductor. When the second switch isoff, the energy is discharged into the primary inductor, charging theindividual storage unit.

In particular embodiments, the system can include a first pulsegenerator that provides first enable signals to the second switch. Thesecond switch couples the string of storage units to the first secondaryinductor in response to the first enable signals, resulting in energybeing transferred from the string of storage units to the individualstorage unit.

The system can also include a second pulse generator that providessecond enable signals to the first pulse generator. The second enablesignals control the transfer of energy from the string of storage unitsto the individual storage unit at a controllable rate with the firstpulse generator providing first enable signals in response to the secondenable signals.

In particular embodiments, the system can further include a secondsecondary inductor that is coupled to the individual storage unit withthe second secondary inductor having a secondary voltage. A voltagecomparator receives the secondary voltage and a reference voltage at itsinputs. When the secondary voltage is less than the reference voltage,the second pulse generator is activated. When the secondary voltagereaches the reference voltage, the second pulse generator isdeactivated.

In particular embodiments, the power converter can include an upconvertor for transferring energy from the individual storage unit tothe string of storage units and a down convertor transferring energyfrom the string of storage units to the individual storage unit. Inother particular embodiments, the power converter can include a commontransformer that is used as a down convertor to charge the individualstorage unit and as an up convertor to discharge the individual storageunit.

Each storage unit can be a storage cell or a battery module having astring of storage units. A string of one or more storage units cancomprise a battery pack.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 is a diagram illustrating a cell balancing system according toone embodiment.

FIG. 2 is a more detailed diagram illustrating a cell balancing systemaccording to one embodiment.

FIGS. 3A and 3B are conceptual diagrams for illustrating thebidirectional charging operation of the equalization converter accordingto one embodiment.

FIGS. 4A and 4B are schematic diagrams for illustrating the operation ofthe switch matrix having unidirectional switches.

FIG. 5 is a more detailed diagram illustrating a cell balancing systemaccording to an alternative embodiment.

FIGS. 6A and 6B are schematic diagrams for illustrating the operation ofthe switch matrix having bidirectional switches according to oneembodiment.

FIG. 7A-7D are flow charts illustrating a method of operating the cellbalancing system according to one embodiment.

FIG. 8A is a schematic diagram illustrating the operation of a boostconverter according to one embodiment.

FIG. 8B is a schematic diagram illustrating the operation of a buckconverter according to one embodiment.

FIG. 9 is a diagram of a cell balancing system having reduced switchblocking requirements according to one embodiment.

FIGS. 10 and 11 illustrate particular embodiments for reducing switchvoltage blocking requirements utilizing the unidirectional andbidirectional switch matrix configurations respectively.

FIG. 12 is a diagram illustrating a cell balancing system according toan alternative embodiment.

DETAILED DESCRIPTION OF THE INVENTION

A description of preferred embodiments of the invention follows.

FIG. 1 is a diagram illustrating a cell balancing system according toone embodiment. The cell balancing system 1 includes a battery pack 100that further includes a string of series-connected storage cells 105 forstoring energy. The battery pack 100 serves as a source to power, forexample, a Hybrid Electric Vehicle (HEV) motor 200. The battery pack 100may also receive charge from the HEV motor 200 when it is driven as agenerator either by the HEV engine or by vehicle kinetic and potentialenergy during braking operations.

The battery pack 100 is further coupled to a switch matrix 300, suchthat each of the storage cells 105 in the pack 100 may be individuallyselected and electrically coupled to an equalization converter 400 formonitoring of voltage, current, and temperature and for equalization ofthe individual cell state of charge toward a target state of charge. Inparticular embodiments, the switch matrix 300 may be an assembly ofsemiconductor switches, facilitating integrated circuit design andreduced fabrication costs.

A control unit 500 directs the switch matrix 300 in selectively couplingindividual cells to the equalization converter 400 and monitors thestate of charge of the individual cells 105 through communication withthe equalization converter 400. According to one embodiment, the controlunit 500 is a micro-controller with on-chip analog-to-digital converter(ADC) of modest resolution (e.g., 10 bits) and a conversion rate of, forexample, 1000 per second. When the state of charge of an individual cellis different than a target state of charge, the control unit 500 directsthe equalization converter 400 to transfer energy between the individualcell and the string of cells 100, such that the state of charge of theselected cell converges toward a target state of charge.

For example, when the state of charge of a selected cell is greater thanthe target, the cell is incrementally discharged by directing theequalization converter 400 to draw current from the cell and to returnthe charge back to the battery pack 100 via connection 401. Conversely,when the state of charge of the selected cell is less than the target,the cell is charged by directing the equalization converter 400 to drawcurrent from the battery pack 100 via connection 401 and to deliver itto the selected cell. State of charge may be determined from a voltagemeasurement under a fixed load using a method depicted in FIGS. 7A-7Dand subsequently described. In such embodiments, an individual cell maybe charged or discharged in a non-dissipative manner.

According to alternative embodiments, the current drawn (i.e.,discharged) from the selected cell may be transferred to anothernon-dissipative load. A load is non-dissipative if at least 80% of theenergy through that element is conserved. A non-dissipative load caninclude an intermediate energy storage buffer (e.g., a capacitor or anauxiliary power supply) for subsequent charging of the string of cellsor an individual cell. According to further alternative embodiments, thenon-dissipative load may also include another storage cell. Additionalswitches and equalization converters may need to be incorporated fortransferring energy to the other storage cell.

State of charge (SOC) is a measure that characterizes electrochemicalstates of a cell after a charging or discharging process. For example,assume that the full capacity of a cell is ‘z’ Ah (Ampere-hour). Afterdischarging 0.5 z Ah capacity from the fully charged state, the state ofcharge of the cell is 50%. Likewise, after discharging 0.25 z Ahcapacity from the fully charged state, the state of charge is 25%, andso on.

The state of charge of a cell can be correlated to cell open circuitvoltages through electrochemical titration of the cell. As anapproximation, a cell's state of charge is often correlated to cellvoltages through charging or discharging voltage profiles at very lowcharging or discharging rates. Since cell state of charge reflect theelectrochemical state, it is important to equalize a cell's state ofcharge in order to enhance cell life and cell safety.

According to particular embodiments of the invention, the state ofcharge is measured by measurement of voltage, current, and impedance.The state of charge is equalized by correcting for the voltagedifference that is due to the load and then comparing voltages ofindividual cells.

In prior art techniques, charge is measured and used as an indicator ofcell equalization. However, charge, which is measured in Ampere-hour(Ah), is quite different from state of charge (SOC). It is notsufficient to only measure charge in order to equalize cells, becausecells having the same charge may not have the same state of charge.

For example, assume two cells have a capacity of 20 Ah. When the cellsare charged to 20 Ah, both have a 4.2V potential. Assuming that fullcell capacity can be utilized, the cells can be equalized to a state ofcharge of 50% by equalizing the charge of the cells to 10 Ah, resultingin a certain voltage less than 4.2V.

However, assume further that the two cells degrade differently, suchthat the first cell can only achieve a maximum capacity of 18 Ah and thesecond cell can only achieve a maximum capacity of 15 Ah in theirrespective degraded state. When both cells are charged to their maximumcapacities, both have a voltage potential of 4.2V. In this case, if thetwo cells are equalized to an equilibrium charge of 10 Ah, the firstcell would be discharged by 8 Ah and the second cell would be dischargedby 5 Ah. This results in the cells having different states of charge(i.e., 56% and 67% respectively), and thus two quite different cellvoltages. Hence the cells are not considered equalized.

Therefore, the method of equalizing cell charge does not become reliablefor the degraded cells or for cells that are not equivalent in nature.This inability to equalize state of charge leads to low battery life andsafety hazards. The low life is caused by potential deep discharge orovercharging, resulting from the inability to determine state of charge.Further, and especially for lithium-ion batteries, the overcharge orover-discharge may cause thermal runaway due to possible formation ofdendrites that can cause internal shortage of the cells.

According to particular embodiments of the invention, the impedance of acell, and therefore the state of charge, can preferably be measured byemploying a test current coming from or into the cell and measuring thevoltage in response. The cell resistance is determined by dividing themeasured cell voltage by the measured cell current. Additionalinformation about the more complex cell impedance can be determined bymeasuring the resistance change over time in response to a constantcurrent or a current profile.

The test current can be a controlled current that can be programmed bythe controller to take a current sample from or supply a current sampleto the cell in a non-dissipative manner via the converter. The testcurrent can also employ the actual load current going into or out of thestring of cells as the vehicle brakes or accelerates.

In the case of the controlled current, the amplitude and duration of thecurrent pulse can be tailored to optimize the impedance measurement. Inthe case of a load current due to vehicle motion, the measurement ofimpedance may not be optimized, but it can occur during normal vehicleoperation.

Both types of test current impedance measurement may be combined ifnecessary. For example, if the vehicle is drawing only a light load fromthe stack that is not sufficient to accurately measure the impedance ofa particular cell, the converter may be programmed to draw just enoughextra current from the cell to provide an accurate measurement.

For batteries, when determining internal impedance of the cell, thevoltage and current are preferably measured during a fast pulse of thesystem. This impedance is only due to the so-called internal impedanceand not due to other polarization effects in the battery. This is alsothe impedance that is used to calculate the voltage that corresponds toa certain state of charge, by correcting for the load applied to thebattery during operation.

FIG. 2 is a more detailed diagram illustrating a cell balancing systemaccording to one embodiment. In this embodiment the switch matrix 300includes an assembly of 2(N+1) unidirectional switches (not shown) whereN is the number of cells in the battery pack 100. In order toelectronically couple an individual cell to, for example, theequalization converter 400, the control unit 500 energizes a pair ofunidirectional switches that provides a conduction path between theconverter 400 and an individual cell 105. In particular, a firstselected pair of switches provides a conduction path for charging thecell and a second selected pair of switches provides a conduction pathfor monitoring and discharging the cell. Implementation of switch matrix300 with unidirectional switches is depicted in FIGS. 4A and 4B andsubsequently described.

The equalization convertor 400 is used to transfer energy between anindividual cell and the string of cells 100 when the state of charge forthe selected cell is different than the target state of change. Thecontrol unit 500 provides control signals directing the converter 400 totransfer energy into a selected cell from the string of cells 100 or totransfer energy out of the cell back into the string of cells 100. Thecontrol unit 500 also provides control signals for limiting current toand from the cell and for identifying a target state of charge (e.g.,cell voltage).

The equalization converter 400 includes a buck converter 412, alsoreferred to as a down converter. When the state of charge of anindividual cell is less than a target state of charge, the control unit500 directs the buck converter 412 to transfer energy from the string ofcells to charge the individual cell.

According to one embodiment, the buck converter 412 may be agalvanically isolated buck converter circuit, which can accept chargefrom a pack of series-connected cells or a battery module of cells(e.g., a 400-volt battery pack of five 80-volt modules) and deliver itto a selected cell. The buck converter input voltage range can bematched to the maximum voltage produced by a string of cells in abattery module (e.g., 80 volts) or the entire battery pack itself (e.g.,400 volts). The buck converter output can be matched to the voltage ofan individual cell. The converter output may also have its voltage andcurrent limits programmable by the control unit 500.

The buck converter power rating may be determined by the maximumequalization charge current demand. For example, the peak power ratingof a buck converter providing a maximum equalization charge current of10 Amperes delivered at 4 Volts is 40 Watts, resulting in the size andcost of the circuit components being relatively small. The buckconverter may also supply a small amount of control power necessary tooperate cell monitoring and equalization control circuits.

In one particular embodiment, the buck converter is a Beta Dyne 40 WPower Watt™ DC/DC Converter. In another particular embodiment, the buckconverter is implemented as a combined buck/boost converter, which canbe enabled to perform either function. A boost converter returns chargeto the pack. The operation of a buck converter according to oneembodiment is illustrated in FIG. 8B.

The equalization converter 400 further includes a boost equalizationconverter 414, also referred to as an up converter. When the state ofcharge of the individual cell is higher than the target state of charge,the control unit 500 directs the boost converter 414 to transfer energyfrom the selected cell to charge the string of cells 100.

According to one embodiment, the boost converter 414 may be agalvanically isolated boost converter circuit which can accept chargefrom a selected cell and return it to a pack of cells or a module ofcells (e.g., a 400 volt battery pack employing five 80-volt modules).Its output provides a current source with adequate voltage to assurecharge acceptance at the maximum desired rate over the operating voltagerange of the pack or module to which it is connected.

The boost converter may have its output voltage limited to avoidover-voltage when disconnected. The boost converter may have its inputvoltage and current limits determined by the control unit 500. Forexample, the input may be configured to accept charge from the connectedcell over the expected range of cell voltages. The size and cost of thecircuit components of the boost converter may be similar to the buckconverter. The operation of a boost converter according to oneembodiment is illustrated in FIG. 8A.

The cell balancing system further includes a cell voltage and currentmonitor 420, and a current sensor 430. The cell voltage and currentmonitor 420 reports selected cell voltage and current to the controlunit 500 for determining the state of charge of that cell. The cellvoltage and current monitor 420 also observes battery current sensor430. Cell voltage and current data is utilized by the control unit 500for determining a target state of charge dynamically. When each of theindividual states of charge of the cells converges toward the targetstate of charge, the charge will be equalized across the string ofcells, facilitating longer operating and calendar life.

FIGS. 3A and 3B are conceptual diagrams for illustrating thebidirectional charging operation of the equalization converter accordingto one embodiment. At this conceptual level, attention is focused on thegalvanic isolation provided by transformer coupling means (a,b) employedin the buck converter 412 and boost converter 414. Switch mode powersupply circuitry required to interface the DC input and DC outputs withthe transformer is assumed but not shown.

In FIG. 3A, when the buck converter 412 is directed by the control unit500 to transfer energy into selected cell V3, current from the string ofcells 100 is drawn through an inductor element “b,” causing inducedcurrent to be driven from the buck converter 412 into selected cell V3via inductor element “a.” For example, in FIG. 3A, where there are 5storage cells in the string, the voltage across the string of cells mustbe down converted to about the target voltage of the individual cell, orabout ⅕ the string voltage. Assuming all cells have about the sameimpedance and 1 A of induced current is driven into the selected cellfrom the buck converter 412, about ⅕ A will be drawn from the string.The resultant current through the cell being charged will then beapproximately (1 A−⅕ A)=⅘ A of current for charging the selected cell,while ⅕ A of current is discharged from the remaining cells.

Conversely, in FIG. 3B, when the boost converter 414 is directed by thecontrol unit 500 to transfer energy out of the selected cell V3, currentfrom the selected cell is drawn through an inductor element “a,” causinginduced current to be driven from the boost converter 414 back into thestring of cells 100 via inductor element “b.” For example, in FIG. 3B,where there are 5 storage cells in the string and 1 A of induced currentis drawn from the selected cell by the equalization discharge converter414, ⅘A of current (i.e., 1 A−⅕ A=⅘ A) is discharged from the selectedcell, while ⅕ A of current is delivered to the remaining cells forcharging.

FIGS. 4A and 4B are schematic diagrams for illustrating the operation ofthe switch matrix having unidirectional switches. The operation of theswitch matrix permits a cell to be selectively charged or discharged andits voltage to be monitored. For embodiments of the cell balancingsystem, such as illustrated in FIG. 2, an assembly of N cells requires2(N+1) independently controlled unidirectional switches S.

Although not so limited, in the illustrated embodiment theunidirectional switches are MOSFET switches S with each coupled to ablocking diode D. The unidirectional switches are arranged such that byenergizing a pair of switches, a conduction path through the selectedcell is maintained for monitoring and/or transferring energy in or outof the cell. Since switches S are unidirectional, one pair of switchesmay be enabled to selectively charge an individual cell and another pairof switches may be enabled to discharge or monitor the same cell.

For example, in FIG. 4A, it is possible to selectively charge anindividual cell V1 by enabling unidirectional MOSFET switches S9 andS12. Current taken from a battery of cells via the buck converter 412,which is represented by controlled current source 12, flows from switchS9 and diode D9 through cell V1 and returns back to the switch matrix300 by switch S12 and diode D12. In this example, blocking diodes D10and D11 prevent cell V1 from being short circuited through the intrinsicbody diodes of S10 and S11 while S9 and S12 are conducting. For example,the intrinsic body diode bd7 of switch S7 is shown in FIG. 4A. Thatdiode allows conduction from right to left even when switch S7 is gatedOFF. Such conduction would provide a short circuit from the positivenode of V2, through the intrinsic body diode of S7 and the ON switch S9if the diode D7 were not in place.

Similarly, in FIG. 4B, it is possible to discharge an individual cell V1by enabling unidirectional MOSFET switches S11 and S10. Charge returningto the battery of cells via the boost converter 414, which isrepresented by a controlled current sink 11, flows from switch S11 anddiode D11 through cell V1 and returns back to the switch matrix 300 byswitch S10 and diode D10. Alternatively, II may be a current limitedvoltage sink (i.e. one which permits cell discharge to a controllablepotential).

As noted above, a diode in series with each unidirectional MOSFET switchis required to block undesired conduction paths. To provide minimalvoltage drop across the blocking diodes, Shottky diodes may be used.Because the associated current is known, it is possible to compensatefor the blocking voltage drop in charge, discharge, and measurementmodes of operation. The residual non-systematic voltage drop aftercompensation is due to variations in diode properties among devices andcannot be compensated, but it is small relative to the operatingvoltages. This combined charge/discharge capability with a singlecontrolled matrix provides the ability to transfer charge selectivelyamong cells to provide for cell balancing.

According to particular embodiments, the unidirectional switches mayalso be insulated gate bipolar transistors (IGBT). However, MOSFETtechnology avoids the conduction loss associated with the IGBTsaturation voltage drop (Vce sat). MOSFET switches provide low impedancewhen turned ON. In particular, the losses due to drop across the MOSFET“on-resistance” (RDS on) are relatively small and there is low MOSFET“on-resistance” variation among devices and with temperature variation,providing an acceptable level of error in all voltage measurementsthrough the matrix.

Isolated gate drives of the MOSFET switches may facilitateimplementation in the form of an application specific integrated circuit(ASIC). According to one embodiment, the gate drivers are galvanicallyisolated light emitting diode (LED) driven photovoltaic (PV) gatedrivers. The modest turn-ON/turn-OFF speeds achieved by these driversare more than adequate for the intended application.

FIG. 5 is a more detailed diagram illustrating a cell balancing systemaccording to an alternative embodiment. This embodiment is substantiallysimilar in operation to that of FIG. 2 with the exception of theconfiguration and operation of switch matrix 300 and the additionalpolarity selector 440 that is controlled by control unit 500. Thepolarity selector 440 is coupled between the switch matrix 300 and theequalization converter 400.

In particular, the switch matrix 300 includes a bank of N+1bidirectional switches (not shown) where N is the number of cells in thebattery pack 100. In order to electronically couple an individual cellto, for example, the equalization converter 400, the control unit 500energizes a pair of bidirectional switches that provides a conductionpath between the converter 400 and the cell. One pair of switches may beused to selectively charge or discharge an individual cell.

The control unit 500 determines whether to charge or discharge the cellbased on whether the state of charge of the selected cell is greater orless than a target state of charge. When the state of charge is greater,the control unit 500 sends a signal to the equalization converter 400 todischarge the cell. At the same time, the control unit 500 also sends acontrol message to the polarity selector 440 to select the polarity oflinks 445 and 447 which are coupled to the switch matrix 300, asdiscussed in FIG. 6A. The polarity selector may be implemented withMOSFET switches and galvanically isolated LED-PV gate drivers orelectromechanical relays. Other implementations of the polarity selectorknown to those skilled in the art may suffice.

FIGS. 6A and 6B are schematic diagrams for illustrating the operation ofthe switch matrix having bidirectional switches according to oneembodiment. The operation of the switch matrix permits a cell to beselectively charged or discharged and its voltage to be monitored. Notethat for this embodiment, a pair of unidirectional MOSFETs connected“back-to-back” comprises a single bidirectional switch. This will beexplained in detail in FIG. 6B.

For embodiments of the cell balancing system, such as illustrated inFIG. 5, an assembly of N cells requires (N+1) independently controlledbidirectional switches. Thus, the number of switching components isreduced from implementations that utilize unidirectional switches. Thisreduction also corresponds to a reduction in the number of gate driversthat are required. The bidirectional switches are arranged such that byenergizing a pair of switches, a conduction path through the selectedcell is maintained for monitoring and/or transferring energy in or outof the cell.

Since switches are bidirectional, a single pair of switches may beenabled to both charge or discharge an individual cell. For example, inFIG. 6A it is possible to selectively charge cell V5 by enabling switchM1/M2 and switch M3/M4. To charge the cell, the equalization converter400 causes current to flow in a direction from switch M1/M2, chargingcell V5 and exiting through switch M3/M4 back to the cell balancingcircuitry. In particular, the internal resistance of the equalizationcircuit 400 is represented by a resistor and the string of cells 100 isrepresented by a battery symbol.

Similarly, in order to selectively charge cell V4, switches M5/M6 andM3/M4 must be enabled. However, in order to drive current from theequalization converter 400 into cell V4 for charging, the polarityacross the pair of switches must be reversed. The polarity is set by thecontrol unit 500 sending control signals to the polarity selector 440 toset the polarity across links 445 and 447. The selection of polarity isbased, for example, on the particular arrangement of the bidirectionalswitches coupling to the individual cells. Other arrangements ofbidirectional switches may be implemented avoiding the need for thepolarity selector.

It is also possible to selectively discharge cell V5 by enabling thesame bidirectional switches M1/M2 and M3/M4 that were enabled to chargethe cell. To discharge the cell, the equalization converter 400 causescurrent to flow in an opposite direction from M3/M4, discharging cellV5, and exiting through switch M1/M2 back to the cell balancingcircuitry. The same polarity set for charging may be used fordischarging.

However, in order to selectively discharge cell V4, switches M5/M6 andM3/M4 are enabled and the polarity across the pair of switches must bereversed. Thus, the equalization converter 400 is able to withdrawcurrent from cell V4, such that current flows from switch M5/M6,discharging cell V4 and exiting through switch M3/M4 back to the cellbalancing circuitry.

FIG. 6B is a schematic diagram of a bidirectional switch according toone embodiment. In this embodiment, the bidirectional switch 305 is apair of discrete MOSFET switches (M1, M2) having a common gate 305 g andsource 305 s. To turn on the bidirectional switch, a voltage is appliedto the gate 305 g. According to one embodiment, the gate is driven bygalvanically isolated LED photovoltaic (PV) gate drivers. Current flowsin a direction determined by the equalization converter 400 and thepolarity applied at terminals d1 and d2 as applied by links 445 and 447of the polarity selector 440.

With this configuration, a forward biased intrinsic body diode, bd1 orbd2, does not introduce a voltage drop to compromise cell voltagemeasurement accuracy because current flows principally through thechannel of the associated MOSFET device and the voltage drop across itschannel resistance is much lower than a diode drop, and thus dominant.Moreover, when the bidirectional switch gate drive is off (i.e., zero orclose to zero volts), the MOSFET switch pair blocks conduction in bothdirections thus avoiding need for the blocking diodes required for theunidirectional switch embodiment depicted in FIG. 4A and FIG. 4B. Thus,the illustrated embodiment of the switch provides low voltage dropswitching with lower power dissipation and high accuracy in cell voltageand state of charge estimation, due to lower contaminating voltage dropsin the circuit.

According to particular embodiments, the bidirectional switches may beimplemented with insulated gate bipolar transistor (IGBT) devices orelectromechanical relays, or Semiconductor Assisted Relay Switches. IGBTswitches do not have an intrinsic reverse diode and provide onlyunidirectional conduction. Hence, if parallel “back-to-back” IGBTswitches are utilized to implement the bidirectional switch previouslydescribed, each IGBT switch must contain series blocking diodes toprevent reverse current.

While bidirectional IGBT switches are applied in power switchingapplications, the conducting IGBT and reverse diode introducecontaminating voltage drops which are highly undesired here. MOSFETswitches provide low impedance when turned ON, and thus are generallypreferred for this reason. However, where voltage and current ratingrequirements cannot be affordably met by MOSFET switches, IGBTembodiments may be viable provided that compensation for the IGBT anddiode conduction voltage drops is provided.

If electromechanical relays can attain suitable cycle life and theirsize and weight are not objectionable, they too may be employed forimplementing the cell selection switch matrix in lieu of MOSFETbidirectional switches, with the advantage of avoiding the need forLED-PV drive isolation and virtual elimination of conduction losses.According to still further embodiments, the bidirectional switches mayalso be implemented with parallel-connected “back-to-back” siliconcontrolled rectifiers (SCRs).

Isolated gate drives of the MOSFET switches and the MOSFET switchesthemselves may facilitate implementation in the form of an applicationspecific integrated circuit (ASIC). Moreover, more than one isolatedgate drive bidirectional switch might be advantageously integrated inone ASIC chip. According to one embodiment, the gate drivers aregalvanically isolated LED driven photovoltaic (PV) gate drivers. Themodest turn ON, turn OFF speeds are more than adequate for the intendedapplication.

FIG. 7A-7D are flow charts illustrating a method of operating the cellbalancing system according to one embodiment. Although not so limited,this method may be implemented as an algorithm that causes the controlunit 500 to perform the following procedure. Referring to FIG. 7A, theprocess starts at 1000. At 1020, the status of the motor is checked. Ifthe motor is off, the method returns to 1000 and waits to poll the motorstatus again. If the motor is started, a cell number n is initialized to1 at 1040 to select the first cell of the battery pack 100. At 1060, theprocess calls subroutine “READ IMPEDANCE.”

Referring to FIG. 7B at 2000, subroutine “READ IMPEDANCE” is started. At2020 a pair of switches is energized, connecting the selected cell(i.e., cell n=1). At 2040, the open circuit voltage (Vopen) of theselected cell is read. At 2060, a load is applied to the selected cell,causing current to flow from the selected cell and the cell voltage todrop. The applied load may be the boost converter 414. By using theboost converter 414, energy from the cell is not lost, as compared witha dissipative load. Rather, the boost converter 414 recharges thebattery 100 with the current discharged from the cell.

At 2080 the loaded voltage (Vload) and current (Icell) of the selectedcell are read. At 2100 the impedance of the selected cell is calculatedand stored in variable Rn2. The cell impedance Rn2 is calculated as aratio of the voltage difference between the loaded voltage (Vload) andthe open circuit voltage (Vopen) divided by the cell current (Icell).The previously saved cell impedance value is stored in variable Rn1.

Since cell impedance can vary due to the dynamic usage of the batterypack, the steps 2060 and 2080 may be repeated multiple times in order toobtain a more accurate determination of cell impedance. For example,according to one embodiment, the boost converter may be operated suchthat constant amplitude current pulses (i.e., DC pulses) are withdrawnfrom the cell at fixed or varied time intervals, as discussed inreference to FIG. 8A. Whenever a DC pulse is drawn from the cell, theloaded voltage may be measured, resulting in a number of loaded voltagemeasurements. Whenever a DC pulse is not being drawn from the cell, theopen circuit voltage across the cell may be measured, resulting in acorresponding number of open circuit voltage measurements. The voltagedifference between the open circuit voltage and the voltage under loadis referred to as a voltage ripple. From the voltage ripples andcorresponding current pulses obtained over a period of time, a moreaccurate cell impedance may be inferred. The resulting cell impedancevalue may then be stored in variable Rn2.

According to a further embodiment, the applied load may be a buckconverter operated such that constant amplitude current pulses (i.e., DCpulses) can be inserted into the cell at fixed or varied time intervalsto infer cell impedance in a similar manner.

At 2120 the applied load is removed. At 2140 the change in cellimpedance (ΔR) between the present and previous cell impedance values(Rn2, Rn1) is calculated. At 2160 it is determined whether the presentvalue Rn2 has changed by 50% or more from the previous value Rn1. If so,a RETRY command is issued at 2180 causing the process to loop back to2040 to recalculate the present cell impedance Rn2. If not, the previousimpedance value Rn1 is replaced with the present value Rn2 at 2200. At2220 the selected pair of switches is de-energized, decoupling theselected cell from the cell balancing circuitry. At 2240, subroutine“READ IMPEDANCE” returns.

Referring back to FIG. 7A at 1080, the cell number ‘n’ is compared withthe total number of cells N in the pack to determine whether all of thepresent cell impedances have been determined. If not, the cell number‘n’ is incremented at 1100 and subroutine “READ IMPEDANCE” is repeateduntil all of the present cell impedances R1, . . . RN have beendetermined.

At 1120 the sum of the present cell impedance values ΣR is calculated.At 1140 the pack voltage, Vp, and pack current, Ip are read. The packcurrent is measured by current sensor 430 and saved. At 1160 a setpointvoltage Vset is calculated by averaging the pack voltage Vp, offset bythe product of the pack current Ip and the sum of the cell impedancesER, over the total number of cells N. In particular, the pack current Iphas a positive value when pack current enters the pack. Conversely, thepack current Ip has a negative value when pack current exits the pack.The set point voltage Vset is referred to as the target state of charge.At 1180, subroutine “SCAN CELLS” is called.

Referring to FIG. 7C, at 3000 subroutine “SCAN CELLS” is started. At3020 the cell number ‘n’ is initialized to 1 to select the first cell inthe pack. At 3040, a load is applied to the circuit. The applied loadmay be the boost converter 414. At 3060 a pair of switches is energized,connecting the selected cell to boost converter 414. Current flows fromthe selected cell and the cell voltage drops. At 3080 the loaded voltageVn and cell current In are read. At 3100 the state of charge of theselected cell is calculated. According to one embodiment, the state ofcharge Vcell is the difference between the loaded voltage Vn and theproduct of the cell current In and the present cell impedance Rn. Inparticular, where the applied load (e.g., boost converter) draws currentout of the cell, the cell current In has a negative value. Conversely,where the applied load (e.g., buck converter) draws current into thecell, the cell current In has a positive value. At 3120, the state ofcharge Vcell is stored in variable Vn.

At 3140 the selected pair of switches is de-energized, decoupling theselected cell from the boost converter 414. At 3160 the cell number ‘n’is compared with the total number of cells N in the pack to determinewhether the state of charge Vcell has been calculated for each cell inthe pack. If not, the cell number ‘n’ is incremented at 3180 and a nextpair of switches is energized to electrically couple the next selectedcell. This is repeated until all the individual cells have beenanalyzed. At 3200, subroutine “SCAN CELLS” returns.

Referring back to FIG. 7A, at 1200 the individual cells are prioritizedfor equalization based on the difference between their state of chargeVcell and the target state of charge Vset. According to one embodiment,the individual cells are sorted for equalization with the individualcell having the largest difference being selected first. Additionally,the amounts of time for charging a cell or returning charge to the packmay be proportioned according to the amount of this difference.According to another embodiment, the individual cells are accessedsequentially for equalization. At 1220 subroutine “EQUALIZE” is called.

Referring to FIG. 7D, at 4000 subroutine “EQUALIZE” is started. At 4060a pair of switches is energized, connecting the selected cell with thehighest priority. At 4080 a load is applied to the selected cell,causing current to flow from the selected cell and the cell voltage todrop. The applied load may be the boost converter 414. At 4000 theloaded cell voltage Vn and cell current In are read. At 4120 the stateof charge Vcell of the selected cell is calculated. According to oneembodiment, the state of charge Vcell is the difference between theloaded voltage Vn and the product of the cell current In and the presentcell impedance Rn. In particular, where the applied load (e.g., boostconverter) draws current out of the cell, the cell current In has anegative value. Conversely, where the applied load (e.g., buckconverter) draws current into the cell, the cell current In has apositive value. After calculating the state of charge Vcell, the appliedload is removed at 4140. At 4160 the state of charge Vcell of theselected cell is compared with the target state of charge Vset.

If the state of charge Vcell is less than target state of charge Vset,the buck converter 412 is applied to the selected cell at 4180. At 4200the charge time is monitored as determined by prioritization, and theapplied voltage is monitored at 4220. If the charge time expires, theequalization charger circuit is removed at 4340. If the charge time hasnot expired, but the voltage from the buck converter that is applied tothe selected cell is too high, the charging voltage is decreased at4240.

If the state of charge (Vcell) is greater than target state of charge(Vset), the boost converter 414 is applied to the selected cell at 4260.At 4280 the discharge time is monitored as determined by prioritization,and the applied voltage is monitored at 4300. If the discharge timeexpires, the boost converter 414 is removed at 4340. If the dischargetime has not expired, but the voltage being applied from the selectedcell to the input of the boost converter is too low, the dischargecurrent is decreased at 4240.

If the state of charge Vcell is equal to target state of charge Vset,within a pre-established deadband, the equalization converter 400 isremoved from the selected cell at 4340. At 4360 the selected pair ofswitches is de-energized, decoupling the selected cell from the cellbalancing circuitry. At 4380 subroutine “EQUALIZE” returns.

Referring back to FIG. 7A, at 1240 the equalize time is compared with apre-determined interval (e.g. 5 minutes). If the equalize time does notexceed the pre-determined interval, the routine loops back to 1180 torepeat subroutine “SCAN CELLS” in preparation for another equalization.If the pre-determined interval has been exceeded, the temperatures ofthe individual cells are scanned at 1260. At 1280 each of the scannedtemperatures T is compared to a pre-established high temperature limit,Tlim. If any of the scanned temperatures T is greater than temperaturelimit Tlim, the system is shut down at 1320. If not, a determination ismade at 1300 whether the temperature of any cell has changed (|ΔT|) bymore than a prescribed increment Tc (e.g., 5 degrees Celsius). If not,the routine loops back to 1180 to repeat subroutine “SCAN CELLS” inpreparation for another equalization. If so, the routine loops back to1040, resetting the cell number to 1 and repeating the process atsubroutine “READ IMPEDANCE”.

Embodiments of the equalization converter 400 may include separate orintegrated buck and boost power converters to monitor the condition ofan individual, selectable, cell in a series string of cells. Suchembodiments also achieve desired cell equalization, such as charging anindividual cell from the series string (buck mode) or transferringexcess energy from an overcharged cell to the series string (boostmode). In particular, charge may be added or subtracted from anindividual cell in a non-dissipative manner.

With respect to monitoring in general, testing of cell state of chargeinvolves measuring temperature, open circuit voltage, and voltage undera fixed load for a certain period of time. Typically, the load currentused is a discharge current into a resistive load, resulting in wastedenergy (i.e., dissipation). If a conventional dissipative load isapplied to a cell, it is normal to keep the measurement time as short aspossible to avoid discharging the cell.

Embodiments of the equalization converter can be used in a cellbalancing system to make the necessary measurements of open circuitvoltage and voltage under load, as well as the change of these voltagesover time, in a non-dissipative manner by monitoring the amplitude ofthe voltage ripple across the cell while transferring energy in either apositive (charge) or negative (discharge) mode without wasting energy.This non-dissipative technique has the advantage of extending thismeasurement time indefinitely, since it is occurring as part of thenormal cell balancing process.

In particular, the equalization converter withdraws or inserts constantamplitude current pulses into a cell. In the time interval between thesepulses, the open circuit voltage is present and measured. At the peak ofthe current pulses, the internal resistance of the cell affects thevoltage in a positive or negative way depending on the direction of thecurrent. The difference between the open circuit voltage and the voltageunder load is the ripple. From multiple determinations of the voltageripple, a more accurate cell impedance can be inferred. The length oftime these current pulses are applied is the charge or discharge time.

In addition, low resistance bi-directional cell selector switches thatconnect the equalization converter to an individual cell are essentiallylossless connections for monitoring the ripple across the cell.

With respect to equalization of cell state of charge, the equalizationconverter 400 removes energy in integer “packets” from an individualcell in a series string of storage cells (e.g., lithium ion cells) andtransfers that energy to the series string as a whole or vice versa. Therate of transfer of these packets represents the power beingtransferred. A timer produces a packet by charging the primary side ofan inductor/transformer for a period of time T1. A second timer controlsthe number of these packets produced per second at a rate of 1/T2.Immediately following the period T1, the energy is transferred to thebattery string via the discharge of the energy stored in theinductor/transformer. The primary and secondary of the converter areelectrically isolated so that the primary can be switched to anyarbitrary cell. The size of the energy packet is controlled by varyingT1, and the power by varying the time T2.

FIG. 8A is a schematic diagram illustrating the operation of a boostconverter according to one embodiment. In particular, the boostconverter enhances monitoring of the state of charge within a cell bywithdrawing fixed amplitude current pulses from the cell and observingthe resultant voltage “ripple.” The amplitude of the resulting ripplevoltage can be used to infer the impedance of the cell. This techniqueprovides a more accurate indication of state of charge than the averagecell voltage taken alone. The current pulses are “up-converted” to ahigher voltage and returned to the series string with only negligiblelosses. This approach is an improvement over prior art equalizationmethods that place a dissipative load on selected cells, thus wastingtheir energy.

In more detail, a pulse generator T1 generates pulses of fixed lengthT1, that are used to close switch S1 connecting the cell to the inductorLp. Current from the cell then charges the inductor (Lp) with atriangular shaped current ramp reaching a peak current (Ipp) of:

Ipp=Vcell*Lp/T1

The resulting pulse of energy (Ep) is stored in the magnetic field ofthe inductor. The pulse energy is: Ep=½Lp*Ipp ². When pulse T1transitions low, switch S1 is turned off and the magnetic fieldcollapses. This causes the energy to discharge into the transformersecondary Ls. The peak current in the secondary winding (Ips), assumingnegligible losses, is:

Ips=(2*Vbat/Ls)½

This secondary peak current discharges into the output capacitor C1 intime T3, where:

T3=(Vbat*Ls)/Ips

It follows that the power output is proportional to the number of T1pulses per second, or 1/T2. When the secondary voltage across inductorLs2 reaches a voltage determined by the voltage reference (Ref), thepulse generator T2 stops generating T1 pulses since the Enable signal isremoved from the pulse generator T2. Ideally, the second secondary hasthe same number of turns as the first secondary Ls and thereforereflects the secondary voltage across the isolation barrier.

FIG. 8B is a schematic diagram illustrating the operation of a buckconverter according to one embodiment. In particular, current is drawnfrom the battery stack 100 and delivered to an individually selectedcell 105. By adding switch S2 on the secondary side and diode D1 on theprimary side of the circuit in FIG. 8A completes the conversion to abuck converter. Not shown in FIG. 8B is isolation circuitry to driveswitch S2 across the isolation barrier. Additionally, minor adjustmentsto the voltage regulator and timer portions may also be necessary.

As shown, a pulse generator T1 generates pulses of fixed length T1 thatare used to close switch S2 connecting the string of cells to theinductor Ls . Current from the string then charges the inductor (Ls)with a triangular shaped current ramp reaching a peak current (Isp). Theresulting pulse of energy (Ep) is stored in the magnetic field of theinductor Ls.

When pulse T1 transitions low, switch S1 is turned off and the magneticfield collapses. This causes the energy to discharge into the inductorLp. This secondary peak current discharges into the output capacitor C1in time T3. It follows that the power output is proportional to thenumber of T1 pulses per second, or 1/T2.

When the voltage across inductor Ls2 reaches a voltage determined by thevoltage reference (Ref), the pulse generator T2 stops generating T1pulses since the Enable signal is removed from the pulse generator T2.Ideally, inductor Ls2 has the same number of turns as inductor Ls andtherefore reflects the voltage across the isolation barrier.

Both diodes D1 and the secondary rectifier diode can be considered partof the switches S1 and S2, as they are the bulk substrate “intrinsic”diodes of N-channel FETS. These intrinsic diodes are now available intheir companion FETS as high speed diodes and improve the switchingefficiency of the circuit. If the Boost circuit is built with an Nchannel FET as the rectifier on the secondary side, then no additionalpower components are needed to convert it to a buck circuit withassociated savings in space and parts cost.

FIG. 9 is a diagram of a cell balancing system having reduced switchblocking voltage requirements according to one embodiment. Due to themodular nature of embodiments of the cell balancing circuitry, themaximum voltage accommodated by the switch matrix switching devices maybe limited to attainable and affordable ratings by connecting multiplecell balancing circuits in series, providing one for each module ofcells constituting the HEV battery pack.

In this embodiment, multiple battery module units, or modules, 100 a,100 b form the battery pack 100 having a total voltage potential Vp.Each battery module is coupled to a respective equalization converter400 a, 400 b by a corresponding switch matrix 300 a, 300 b. Theequalization coverters 400 a, 400 b are coupled in series, such thateach converter is coupled across the terminals of a correspondingbattery module 100 a, 100 b. This configuration reduces the switchblocking voltage requirement to Vp/M where M is the number of batterymodules connected in series. In the illustrated embodiment, there areM=2 modules, resulting in each unit balancing the state of charge of astring of cells of the respective battery module 100 a, 100 b with theswitch blocking voltage requirement reduced in half. By reducing theswitch blocking voltage, cost benefits exist due to the ability to uselower voltage rated switch components.

A master control unit 600 directs local control units 500 a, 500 b inselectively monitoring and equalizing (i.e. charging and discharging)individual cells of the battery modules 100 a, 100 b as previouslydescribed. The master control unit 600 and the control units 500 a, 500b may communicate over a galvanically isolated serial data communicationlink. In particular, the master control unit 600 broadcasts pack currentand equalization setpoint values (i.e., target state of charge) to thelocal control units 500 a, 500 b. Likewise, the local control units maycommunicate diagnostic or prognostic status information to the mastercontrol unit 600, such as voltage, current, and temperature, from themodules.

FIGS. 10 and 11 illustrate particular embodiments for reducing switchvoltage blocking requirements utilizing the unidirectional andbidirectional switch matrix configurations respectively. Suchembodiments result in the individual cells of a corresponding batterymodule being equalized to a target state of charge. However, the targetstate of charge in one battery module 100 a may be different from thetarget state of charge for another battery module 100 b.

Due to the modular nature of the cell balancing circuitry, manydifferent configurations may be employed. For example, it is possible tohave a configuration where one or more battery modules in a battery packhave their states of charge being balanced by embodiments of the cellbalancing system, while other battery modules within the same batterypack are balanced using other known cell balancing systems. Furthermore,it is also possible to employ a nested control system where one or morebattery modules in a battery pack have their states of chargeindividually balanced by embodiments of the cell balancing system and asecondary control system for balancing the state of charge across theentire pack. The secondary control system may be an embodiment of thecell balancing system or alternatively a different cell balancingsystem. Thus, the cell balancing may be applied to electrical energystorage units which may be cells or modules of multiple cells.

FIG. 12 is a diagram illustrating a cell balancing system according toan alternative embodiment. In the illustrated embodiment, equalizationconverters 400 a, 400 b are coupled to the terminals of the entirebattery pack of M battery modules, resulting in the individual cells ofeach battery module being equalized to a common target state of charge.However, this configuration requires a switch blocking voltagerequirement that is at least equal to the maximum voltage of the pack.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1-22. (canceled)
 23. A system, comprising: a string of electrical energystorage units having a first end and a second end; and a buck-boostdc-dc power converted coupled simultaneously to a selected portion ofthe string of electrical energy storage units and to the first andsecond ends of the string of electrical energy storage units, the powerconverter being configured to transfer energy bidirectionally betweenthe selected portion of the string of storage units and the end pointsof the string of storage units, wherein the power converter isconfigured to transfer energy at a controllable rate of transfer, andthe power converter comprises: an up-converter configured to transferenergy from the selected portion of the string of storage units to theend points of the string of storage units; and a down-converterconfigured to transfer energy from the string of storage units to theselected portion of the string of storage units.
 24. The system of claim23, wherein the power converter is configured to monitor voltage andcurrent data of the selected portion of the string of storage unitsresulting from a transfer of energy.
 25. The system of claim 23, whereinthe power converter is configured to transfer units of energy betweenthe selected portion of the string of storage units and the end pointsof the string of storage units.
 26. The system of claim 23, wherein thepower converter comprises: a primary inductor; a first secondaryinductor magnetically coupled to the primary inductor; a first switchselectively coupling the selected portion of the string of storage unitsto the primary inductor; and the first secondary inductor coupling to anoutput capacitor; the output capacitor coupled in parallel to the stringof storage units.
 27. The system of claim 23, further comprising: aprimary inductor; a first secondary inductor magnetically coupled to theprimary inductor; a switch selectively coupling the first secondaryinductor to the end points of the string of storage units, andconfigured to transfer energy from the string of storage units to chargethe first secondary inductor when the switch is on, and to dischargeenergy into the primary inductor and charging the selected portion ofthe string of individual storage units when the switch is of.
 28. Thesystem of claim 23, wherein a common transfer is configured to serve asthe up-converter and the down converter.
 29. The system of claim 23,wherein each storage unit is a storage cell.
 30. The system of claim 23,wherein each storage unit is a battery module having a string of storageunits.
 31. The system of claim 23, wherein a battery pack comprises astring of one or more storage units.